1. Field of the Invention
The present invention relates generally to well logging and more specifically it relates to a logging while drilling system for efficiently providing a reliable measurement of various petrophysical properties of subsurface earth formations.
2. Description of the Related Art
Any discussion of the related art throughout the specification should in no way be considered as an admission that such related art is widely known or forms part of common general knowledge in the field.
Well logging has been in use for years. Well logging is commonly utilized by the oil and gas industry to evaluate the subsurface formations (i.e. reservoir) of the earth in order to determine various characteristics (e.g. permeability, composition, porosity, etc.) of the subsurface formations and fluids within the subsurface formations. The reservoir rock (i.e. matrix) usually includes a mixture of quartz, clay, limestone, dolomite, anhydrite and various other minerals. The fluids filling-up the voids or cracks (i.e. porosity) in the reservoir generally include a mixture of water, hydrocarbons such as oil and gas, and various others.
The primary objective of well logging by the oil and gas industry is to identify how much hydrocarbons are in place (i.e. reserves in place) by including a breakdown of how much hydrocarbons can be produced (i.e. recoverable reserves), how much hydrocarbons will be left behind, and how fast the reservoir can be drained and the hydrocarbons produced. For this purpose, wells are drilled and the subsurface formations are penetrated are evaluated.
Engineered ‘mud’ is generally circulated within the wellbore while drilling the well. The mud is injected at the surface inside the drill-pipe and subsequently flows inside the drill-pipe all the way to the drill-bit attached at the bottom of the drill-pipe and wherein the mud then exits the drill-bit from various nozzles and comes back up to the surface through the annular space in-between the outside of the drill-pipe and the wall of the wellbore being drilled (i.e. borehole).
This mud serves many purposes, such as to carry the drilled formations cuttings back to surface, to lubricate and cool the drill-bit and to prevent any formations fluids inflow into the wellbore, by ensuring the hydrostatic pressure of the mud downhole exceeds the pressure of the fluids filling the pore-space of the drilled formations at all times. Because the pressure of the mud inside the wellbore is higher than the pressure of the fluids inside the formations, it is the mud that is forced inside the formations, displacing and flushing away the formations fluids (i.e. a process called invasion).
Whole mud invasion sometimes occurs; however most of the time the pore and pore-throats that make up the voids of the formation are so tiny that the matrix acts like a filter and only a “mud filtrate” (i.e. liquid portion of the mud) invades the subsurface formation. The residual solid contents of the mud (i.e. mud cake) deposit at the interface with the wellbore and build-up over time as invasion progresses thus getting thicker and behaving like a seal to slow-down the invasion process itself. As invasion progresses deeper into the formation, the zone that the mud filtrate reaches into is called “the invaded zone” or also “the flushed zone”, and the zone that has not been contaminated by the invasion process yet is called “the virgin zone”.
Various methods have been utilized to evaluate the subsurface formations penetrated. Two commonly utilized options are either to “core” the wellbore or “log” the wellbore. Coring the wellbore generally includes cutting a cylindrical core within the subsurface formation and recovering the core to the surface to analyze and evaluate the core in dedicated labs. However coring techniques are time-consuming and prohibitive, and cutting a core and bringing it to the surface might modify its characteristics due to different temperature, pressure, confinement configuration at the surface versus downhole and not being able to preserve the original fluid information. In addition, the turn-around-time required to complete a core study, is on the order of months if not years which is inconvenient for quick decision-making purposes.
Logging the wellbore generally includes acquiring various petrophysical measurements over the formations penetrated by the wellbore by way of sondes fitted with various sensors. The sondes are conveyed down the drilled well to collect various types of information and data (i.e. logs). Logging is generally the time efficient way to have information readily available about the well (i.e. the information is generally in electronic format); however it can require advanced interpretation techniques in order to transform various measurements into the desired rock and fluid composition data needed to properly evaluate the subsurface formation.
Various ways in which to log data have been performed in the past. One such way is by utilizing “wireline” logs, wherein the wireline logs were acquired days after drilling a wellbore and they were so called, because the sondes loaded with sensors were lowered deep beneath the surface of the earth and inside the wellbore, using an armored electric wire attached to the surface. The electric wire was used to lower the sensors in the well, transmit power to the sensors, send commands to the tools downhole, retrieve data to the surface and recover the sondes to the surface.
Generally the sensors utilized to generate log data and measurements (e.g. measurements using neutron sources or gamma-ray sources, and neutron or gamma-ray detectors, and nuclear-magnetic-resonance NMR measurements, etc.) only reach a few inches deep (i.e. radially away from the wellbore wall) within the subsurface formation being investigated, wherein the volume being investigated by wireline logs acquired at “wireline time” (i.e. wireline logs acquired days after drilling a wellbore) is generally within the invaded-zone, with the exception of deep-reading electromagnetic resistivity measurements, and with the exception of a few situations encountered sometimes in practice, where the invasion itself does not reach any deeper than a few inches at wireline time.
The invaded-zone and the virgin-zone only share the formation rock (i.e. matrix) and voids (i.e. porosity) in common, but the fluid composition is different in-between both, and there are even situations where the invaded-zone and the virgin-zone do not even share the matrix and the porosity in common, such as in case of chemical interaction in-between the mud-filtrate and one or more of the matrix minerals. Evaluating the invaded zone on its own is therefore generally insufficient, when it is not flawed such as in the situations discussed above, wherein the virgin-zone is the volume of the subsurface formation that is of most interest to the oil and gas industry from a hydrocarbons reserves perspective, and wherein understanding differences and similarities in-between both the invaded-zone and the virgin-zone fluid contents, is necessary for a better understanding of how efficiently mud-filtrate displaced and flushed the original formation fluids, and hence permeability and recoverable reserves estimates.
The deep-reading electromagnetic resistivity measurements reaching beyond the invaded-zone and into the virgin-zone, traditionally provided a useful means to estimate water content in the virgin-zone, and hence the hydrocarbons content in the virgin-zone was worked-out by subtracting the estimated water content in the virgin-zone, from the total porosity computed from logs reading in the invaded zone (and bearing in mind the underlying assumption that the invaded-zone and the virgin-zone shared the porosity in common). However resistivity logs introduced another level of complication of their own, due to the resistivity equations used to convert resistivity into water content, wherein the equations can be very challenging to design, thus making it difficult to extract reliable saturation and connected-porosity information using resistivity data. This is for example the case for carbonate formations and low-resistivity-pay intervals, whereby empirical techniques are extensively used (and these empirical techniques usually involve neural-network and reservoir rock-typing techniques).
Other times, variable formation water salinity, such as but not limited to, water injection situations to maintain reservoir pressure, fresh formation water scenarios, or chemical interaction in-between the mud-filtrate and the formation matrix minerals, can make it virtually impossible to extract reliable water content information from either resistivity or thermal neutron capture cross-section logs (i.e. SIGMA logs) individually, or together simultaneously.
Another more recent method to log data from the wellbore includes “logging-while-drilling”, wherein sensors similar to those used for wireline logging were actually integrated in the drill-pipe close to the drill-bit, wherein the sensors readings had the added benefit of being available almost real-time as the subsurface formations are drilled. Although, invasion is generally shallower while the log data is acquired utilizing logging-while-drilling techniques (rather than while utilizing wireline logging techniques) because the sensors are an integral part of the drill-string and the measurements are made just hours after the earth formation is freshly drilled, invasion still cannot be dismissed, and invasion effects cannot be disregarded, as invasion is still not technically zero.
Thus, logging-while-drilling measurements generally include at least a portion of the invaded zone, and assuming otherwise that logging-while-drilling measurements can be considered representative of the virgin zone only, can naturally lead to erroneous information relating to the subsurface formation. Because of the inherent problems with the related art, there is a need for a new and improved logging-while-drilling system, to remedy the situation thru a new and efficient log data acquisition and interpretation methodology, that fully accounts for the invasion process, by manufacturing two separate log datasets totally immune to invasion, and that respectively embody the invaded zone only and the virgin zone only, thereby enabling a new suite of formation evaluation applications that could not be considered in the past.